Skip to content

Inputs

A transaction's inputs say which UTXOs are being spent. Each input points at a previous output by (transaction_id, index) and carries the script that proves the spender is allowed to claim it.

Types involved

TransactionInput
  previous_outpoint: TransactionOutpoint(transaction_id, index)
  signature_script  (filled at sign time)
  sequence
  sig_op_count
  utxo: UtxoEntryReference   # optional, but you almost always want it set
  • TransactionOutpoint — (transaction_id, index). The pointer to the output being spent.
  • UtxoEntryReference — a cached copy of the spent output: its amount, lockup script, block DAA score, and coinbase flag.
  • signature_script — the unlocking script. Empty ("") at build time; filled when you sign. See Signing.
  • sequence — sequence number. Leave at 0 unless you have a specific protocol-level reason.
  • sig_op_count — number of signature operations this input performs (1 for Schnorr/ECDSA, >1 for multisig). Feeds into mass calculation.

Build an input

From a UTXO dict returned by client.get_utxos_by_addresses(...):

from kaspa import TransactionInput, TransactionOutpoint, UtxoEntryReference

resp = await client.get_utxos_by_addresses({"addresses": [my_address]})
utxo = resp["entries"][0]

inp = TransactionInput(
    previous_outpoint=TransactionOutpoint(
        transaction_id=utxo["outpoint"]["transactionId"],
        index=utxo["outpoint"]["index"],
    ),
    signature_script="",          # filled at sign time
    sequence=0,
    sig_op_count=1,
    utxo=UtxoEntryReference(utxo),
)

Why inputs carry a UtxoEntryReference

Kaspa signs over the spent output's amount and lockup, not just the outpoint. The SDK can't sign correctly without that context, so TransactionInput.utxo attaches it directly — no node round-trip needed.

Consequences:

  • Forgetting utxo=... when building manually breaks signing.
  • A signed transaction can move between processes (offline signer, co-signer, relay) without the receiver needing the source node — every input carries what's needed.
  • The Generator handles this for you. Pass UtxoEntryReferences (or a UtxoContext — see UTXO Context) and it picks and wraps inputs internally.

Selecting which UTXOs to spend

The Generator (see Transaction Generator) handles selection. When building manually, sum input values until the total covers outputs + fee, then route the remainder to a change output. See Outputs → Change.

Reading inputs back

for inp in tx.inputs:
    print(inp.previous_outpoint.transaction_id, inp.previous_outpoint.index)
    print(inp.signature_script_as_hex)   # hex string after signing, None before
    print(inp.sig_op_count, inp.sequence)
    if inp.utxo:
        print(inp.utxo.amount, inp.utxo.script_public_key)